Fix favicon not updating on the client side when changed.

Regression from 1c6a2262b3.

Documented the rational for include the url since we can't really test
this properly.

app/assets/javascripts/discourse.js.es6

@@ -62,7 +62,15 @@ const Discourse = Ember.Application.extend({
   @observes("notifyCount")
   faviconChanged() {
     if (Discourse.User.currentProp("dynamic_favicon")) {
-      const url = Discourse.getURL("/favicon/proxied");
+      let url = Discourse.SiteSettings.site_favicon_url;
+
+      // Since the favicon is cached on the browser for a really long time, we
+      // append the favicon_url as query params to the path so that the cache
+      // is not used when the favicon changes.
+      if (/^http/.test(url)) {
+        url = Discourse.getURL("/favicon/proxied?" + encodeURIComponent(url));
+      }
+
       new window.Favcount(url).set(this.get("notifyCount"));
     }
   },

app/controllers/static_controller.rb

@@ -124,7 +124,7 @@ class StaticController < ApplicationController
     is_asset_path
 
     hijack do
-      data = DistributedMemoizer.memoize("FAVICON#{SiteSetting.favicon&.id}", 60 * 30) do
+      data = DistributedMemoizer.memoize("FAVICON#{SiteSetting.site_favicon_url}", 60 * 30) do
         favicon = SiteSetting.favicon
         next "" unless favicon